I am developing a piece of software that can only run for a specific amount of time based on the registration date.

For example if the user registers for October 24th 2007 then i want to allow him the functionality of the software for the 23rd, 24th, and 25th of October.

How do i get dates which the user cannot tamper with. Windows XP and Windows Vista references would be great.

I am attempting to come up with a scheme on providing these features in the specified time range so I do not have code examples to show. I am in the planning phase, since i am not sure if this is possible or how it can be circumvented I havent explored other methods of doing this, I am very open to suggestions.